1. Field of the Invention
The present invention is related to gyroscopes, and more particularly, to gyroscopes having high sensitivity and high signal-to-noise ratio.
2. Background Art
Vibrational gyroscopes have many advantages over conventional gyroscopes of the spinning wheel type. Thus, a vibrational gyroscope is considerably more rugged than a conventional spinning wheel gyroscope, can be started up much more quickly, consumes much less power and has no bearings which could be susceptible to wear.
A wide variety of vibrating members have been employed in previously proposed vibrational gyroscopes, ranging in shape from a tuning fork to a pair of torsionally oscillating coaxial spoked wheels. However the present invention is particularly concerned with vibrational gyroscopes in which the vibrating member comprises a radially vibrating annular shell, such as a hemispherical bell or a cylinder for example. In such gyroscopes the axis of the annular shell (e.g., the z axis) is the sensitive axis and the shell, when vibrating, periodically distorts in an elliptical fashion with four nodes spaced regularly around the circumference and located on the X and Y axes. Any rotation about the z axis generates tangential periodic Coriolis forces which tend to shift the vibrational nodes around the circumference of the shell and thereby generate some radial vibration at the original nodal positions on the X′ and Y′ axes. The Coriolis force can be calculated based on the relationship Fc=2VxΩ, where Fc is the Coriolis force, V is the linear velocity vector of the mass elements of the resonator (shell) due to fundamental mode vibration, “x” is the vector product, and Ω is the angular velocity vector. Consequently the output of one or more transducers located at one or more of these nodal positions gives a measure of the rotation rate (relative to an inertial frame) about the Z-axis.
This highly symmetrical system has a number of important advantages over arrangements in which the vibrating member is not rotationally symmetrical about the z-axis. Thus, the component of vibration rotationally induced by the Coriolis forces is precisely similar to the driving vibration. Consequently, if the frequency of the driving vibration changes (e.g. due to temperature variations) the frequency of the rotationally induced component of vibration will change by an identical amount. Thus, if the amplitude of the driving vibration is maintained constant, the amplitude of the rotationally induced component will not vary with temperature. Also the elliptical nature of the vibrational distortion ensures that the instantaneous polar moment of inertia about the z-axis is substantially constant throughout each cycle of the vibration. Consequently, any oscillating torque about the z-axis (due to externally applied rotational vibration) will not couple with the vibration of the walls of the shell. Accordingly vibration gyroscopes incorporating an annular shell as the vibrating member offer superior immunity to temperature changes and external vibration.
However in practice, vibrational gyroscopes generally employ piezoelectric transducers both for driving and sensing the vibration of the vibrating member. In cases where a vibrating annular shell is employed, the transducers are mounted on the curved surface of the shell, generally near its rim. Since it is difficult to form a low compliance bond between two curved surfaces, the transducers must be sufficiently small to form an essentially flat interface with the curved surface of the annular shell. The output of the vibration-sensing transducers is limited by their strain capability, so that the sensitivity of the system is limited by signal-to-noise ratio. All these problems become more acute as the dimensions of the annular shell are reduced.
FIG. 1 illustrates how Coriolis forces are used in gyroscopes to measure the speed of rotation. As shown in FIG. 1, a resonator, typically in the shape of a cylinder, designated by 104 in its un-deformed state, is rotated. The vibration modes of the cylinder 104 involve “squeezing” the cylinder along with one of its two axes, thereby forming an ellipse. One of the axes, designated by X, becomes the major axis of the ellipse, and the other one, designated by Y, becomes the minor axis of the ellipse. This is the primary vibration mode of the cylindrical resonator, with the vibration mode designated by 101 in FIG. 1.
In essence, the cylindrical resonator alternates between orthogonal states, shown by 101 in FIG. 1. When the resonator rotates at an angular velocity Ω, a second vibration mode starts to appear, which is designated by 102 in FIG. 1. This is due to Coriolis force vectors 103, which result in a Coriolis force in a combined Coriolis vector 105. Therefore, the added standing vibration wave 102 is oriented at 45° relative to the primary vibration modes 101. The amplitude of the standing wave 102 is related to the angular velocity of the resonator, and is processed electronically to generate a value representative of that angular velocity. It will be appreciated from FIG. 1 that if the rotation of the resonator 104 were counterclockwise (instead of clockwise, as shown in the figure), the orientation of the resulting Coriolis force vector would be at 90° to what is indicated by 105 in FIG. 1, and would be detected accordingly.
As discussed above, conventional Coriolis force gyroscopes typically use a machined resonator cavity, or cylindrical resonator, with a number of piezoelectric elements that are attached to the body of the cylinder. Some of the piezoelectric elements are used to drive the vibration of the cylinders, and others are used to detect the standing wave due to the rotation, indicated by 102 in FIG. 1. A typical arrangement involves eight such piezoelectric elements arranged, equiangularly around the circumference of the resonator 104, such that the major and minor axes of the ellipse (X and Y in FIG. 1) have four piezoelectric elements used to generate the primary vibration mode 101, and four piezoelectric elements arranged along the axes X′ and Y′, used to detect the standing wave 102 due to the Coriolis force.
It is relatively straightforward, using current technology, to machine a very precise resonator 104, to extremely high tolerance. However, the piezoelectric elements are typically glued to the outside of the resonator. The overall structure, therefore, deviates from a perfectly symmetrical structure, since it is extremely difficult to glue the piezoelectric elements with perfect repeatability. Typical dimensions of such structures are on the order of a few millimeters to perhaps a centimeter for the smaller resonators, and larger dimensions for some of the bigger ones. The fact that the perfectly vibrating cylinder of FIG. 1 becomes an asymmetrical structure has a direct effect on the gyroscope sensitivity, and the signal-to-noise ratio, since some of the Coriolis force-driven standing wave 102 and the primary vibration mode 101 begin to overlap, rather than be at a perfect 45° angle to each other.
Such resonators as used in Coriolis vibrational gyroscopes have several disadvantages:
1. locating the piezoelectric element t near a free edge of the resonator acts to dampen the standing wave. This, in turn, leads to a reduction in the Q-factor of the gyroscope, which in turn leads to restrictions on the scaling (of the size) of the gyroscope, and generally, on the sensitivity of the gyroscope.
2. the use in such resonators of piezoelectric elements that are glued to the surface of the resonator leads to an uneven distribution of stiffness in the resonator, when moving in a circumferential direction. This means that the axis of the standing wave does not necessarily coincide with the axes of the piezoelectric elements. This in turn leads to a nonlinearity in the scaling coefficient of the gyroscope, as well as to a reduction in the sensitivity of the gyroscope. This effect is particularly visible at relatively low rates of rotation.
If the bottom of the resonator has a number of openings that are typically evenly distributed in the circumferential direction (and also symmetrical relative to the axis of the resonator), together with a number piezoelectric elements in between the openings, such a construction has the following disadvantages:
1. locating the piezoelectric elements in between the openings does not permit an exact coincidence between the axis of the standing wave and the axes of the piezoelectric elements (in other words, the coordinate system of the two effects do not coincide).
2. mechanical balancing and tuning of the resonator in this case is a fairly complex procedure, although this is generally true of many such CVGs.
A conventional CVG gyroscope is described in U.S. Pat. No. 4,644,793, which include a cylindrical cup as a resonator. The resonator is affixed to a flat flexible plate (a membrane), while the membrane is connected to a ceramic disk on which a set of electrodes is located. Deformation of the membrane, when an AC excitation signal is supplied to the resonator causes radial vibration. When rotating about their axis, the nodes of the radial vibration move on the circumference of the resonator cup due to the action of the Coriolis force. The movement of the nodes is transferred to the membrane on which sensors are located, for example, capacitive sensors, which pickup the movement of the nodes.
Such a device has a number of disadvantages:
1. the use of a membrane in a sensing element, which needs to be clamped down on its outer edge, and loaded with the cylindrical cup on its inner edge, leads to the undesirable effect when non-inertial influences are experienced, such as vibration or shock. In this case, the membrane can resonate at its own frequencies, which can be very similar, or identical, to the primary frequency of the resonator cup. This, in turn, limits the sensitivity of the gyroscope, and can also lead to significant errors in the measurement.
2. placing sensors on the membrane leads to a lack of coincidence of the axis of the standing wave with the axes of the sensors, and also leads to a reduced sensitivity of the gyroscope and a lower accuracy of measurement of the angular velocity Ω.
3. mechanical balancing and tuning of the sensing element is still fairly complex, similar to the other gyroscopes described above.
Accordingly, there is a need in the art for a gyroscope with high precision, high sensitivity and a high signal-to-noise ratio.